Heart rate variability ("HRV") is known to be associated with a healthy heart. Numerous clinical studies have shown that patients with higher levels of heart rate variability have an increased chance of survival. Heart rate variability encompasses both long term HRV and short term HRV. Long term HRV is primarily the result of circadian rhythms and activity associated with sleep and awake cycles. Short term HRV is primarily the result of faster acting physiologic responses to respiration and blood pressure control. It has been demonstrated in clinical studies that both short term and long term HRV are related to likelihood of survival.
The beneficial effects of short term HRV are believed to be related to its modulating effect on the cardiac muscle cells and its ability to disrupt or diminish the evolution of dangerous patterns of electrical activity. One such important pattern is the pattern of electrical alternation in the action potential of the cardiac cells. It has been recognized that, in many patients who are at higher risk for serious cardiac arrhythmias, there may emerge a pattern of local or regional alternation in the action potential. This pattern of alternation is not always present, but often emerges under conditions where the patient's heart experiences an increased demand due to an increased level of physical or mental stress.
The pattern of alternation is referred to as electrical alternans. Electrical alternans can often be measured at the body surface as a subtle beat-to-beat change in the repeating pattern of an electrocardiogram (ECG) waveform. Alternans can be indicative of electrical instability of the heart and increased susceptibility to sudden cardiac death. Alternans results in an ABABAB . . . pattern of variation of waveform shape between successive beats in an ECG waveform. An overview of electrical alternans is given by Rosenbaum, Albrecht and Cohen in "Predicting sudden cardiac death from T wave alternans of the surface electrocardiogram: promise and pitfalls.", Journal of Cardiovascular Electrophysiology, Nov., 1996, Vol. 7(11), pages 1095-1111, which is incorporated by reference.